A wide variety of unmanned aerial vehicles are used for commercial, scientific, and recreational purposes. For example, an aerial vehicle with an attached image capture device (e.g., a cameras and/or a video camera) may allow for the capture of images or video footage from vantage points that would otherwise be inaccessible to a photographer. Aerial vehicles may operate autonomously, be controlled by a user, or be controlled by a hybrid approach.
An autonomous or a remote controlled aerial vehicle may include a magnetometer compass that measures the earth's magnetic field, by which the orientation (e.g., the heading) of the aerial vehicle may be determined. The output of the sensors a magnetometer may be distorted by various factors, such as manufacturing inconsistencies, thermal effects on the sensors, cross-axis effects, hard-iron interference (e.g., magnetic fields produced by permanent magnets or magnetized iron/steel that are in the vicinity of the magnetometer), and soft-iron interference (e.g., magnetic fields produced by current-carrying traces or wires). For this reason, in order to accurately determine the orientation of the aerial vehicle, it is sometimes necessary to calibrate the compass.
Calibration procedures for a magnetometer are known. Generally, calibration involves manually rotating the aerial vehicle horizontally (i.e., rotating about a vertical axis). Often, the aerial vehicle is rotated horizontally multiple times with the aerial vehicle oriented differently each time. This method of calibrating the magnetometer generally performs well in areas where the earth's magnetic field has a substantial horizontal component. For example, a calibration method where the aerial vehicle is rotated horizontally would perform well in Havana, Cuba where the inclination (i.e., the angle between the magnetic field vector and a horizontal plane) is about 52° 23′. If the inclination is nearly horizontal, the direction of the magnetic field relative to the aerial vehicle, as measured by the magnetometer, will change significantly when the aerial vehicle is rotated horizontally about a vertical axis.
Although this type of calibration method may perform suitably where the direction of the earth's magnetic field is nearly horizontal (i.e., when the magnetic inclination is nearly zero), it may not be effective in areas where absolute value of the magnetic inclination is large (i.e., when the magnetic field has a large vertical component). This is because rotating the aerial vehicle horizontally will not greatly change the direction of the magnetic field relative to the aerial vehicle. Thus, in a place like Ísafjörður, Iceland, where the magnetic inclination is 76° 41′, or in Alert in the Qikiqtaaluk Region of Canada, where the magnetic inclination is 86° 20′, the calibration procedures disclosed in the prior art may perform poorly. As another example, the magnetic inclination at Campbell Island, New Zealand, is about −75° 26′, which may also cause inaccurate calibration when the calibration systems of existing aerial vehicles are used.